A mobile communication system is required to compress speech signals in low bit rates for effective use of radio resources. Further, improvement of communication speech quality and realization of a communication service of high actuality are demanded. To meet these demands, it is preferable to make quality of speech signals high and encode signals other than speech signals, such as audio signals in wider bands, with high quality. For this reason, a technique of integrating a plurality of coding techniques in layers is regarded as promising.
For example, this technique refers to integrating in layers the first layer where input signals according to models suitable for speech signals are encoded at low bit rates and the second layer where error signals between input signals and first layer decoded signals are encoded according to a model suitable for signals other than speech (for example, see Non-Patent Document 1). Here, a case is shown where scalable coding is carried out using a standardized technique with MPEG-4 (Moving Picture Experts Group phase-4). To be more specific, CELP (code excited linear prediction) suitable for speech signals is used in the first layer and transform coding such as AAC (advanced audio coder) and TwinVQ (transform domain weighted interleave vector quantization) is used in the second layer when encoding residual signals obtained by removing first layer decoded signals from original signals.
By the way, the TwinVQ transform coding refers to a technique for carrying out MDCT (Modified Discrete Cosine Transform) of input signals and normalizing the obtained MDCT coefficient using a spectral envelope and average amplitude per Bark scale (for example, Non-Patent Document 2). Here, LPC coefficients representing the spectral envelope and the average amplitude value per Bark scale are each encoded separately, and the normalized MDCT coefficients are interleaved, divided into subvectors and subjected to vector quantization. Particularly, the spectral envelope and average amplitude per Bark scale are referred to as “scale factors,” and, if the normalized MDCT coefficients are referred to as “spectral fine structure” (hereinafter the “fine spectrum”), TwinVQ is a technique of separating the MDCT coefficients to the scale factors and the fine spectrum and encoding the result.
In transform coding such as TwinVQ, scale factors are used to control energy of the fine spectrum. For this reason, the influence of scale factors upon subjective quality (i.e. human perceptual quality) is significant, and, when coding distortion of scale factors is great, subjective quality is deteriorated greatly. Therefore, high coding performance of scale factors is important.    Non-Patent Document 1: “Everything about MPEG-4” (MPEG-4 no subete), the first edition, written and edited by Sukeichi MIKI, Kogyo Chosakai Publishing, Inc., Sep. 30, 1998, page 126 to 127.    Non-Patent Document 2: “Audio Coding Using Transform-Domain Weighted Interleave Vector Quantization (TwinVQ),” written by Naoki IWAKAMI, Takehiro MORIYA, Satoshi MIKI, Kazunaga IKEDA and Akio JIN, The Transactions of the Institute of Electronics, Information and Communication Engineers. A, May 1997, vol. J80-A, No. 5, pp. 830-837.